For almost a decade, metallic conductors, which are insulated with an irradiation cross-linked polymeric plastic material, have been used in the communications industry. These kinds of insulated conductors have been found to be very suitable for applications in which the insulation is required to have relatively high solder heat, abrasion and cut-through resistance values. A metallic conductor such as copper, for example, is insulated with a polyvinyl chloride composition which contains a cross-linkable monomer. Then the insulation is irradiation cross-linked to provide the requirements which are identified hereinbefore. Such a product is shown in U.S. Pat. No. 3,623,940 which issued on Nov. 30, 1971 in the names of H. M. Gladstone and L. D. Loan.
Several problems have developed with respect to the irradiation cross-linked insulation which has been described hereinbefore. In a process for making the insulated conductor, the metallic conductor is preheated and is enclosed in an extrudate comprising the cross-linkable plastic material. It has been found that the minimum temperature to which the metallic conductor in the form of wire must be preheated is especially critical. If the temperature of the metallic conductor is too low, the temperature differential between it and the extrudate will cause rapid cooling and result in thermal stress cracking of the plastic material. Should the preheat temperature be too high, the adhesion between the metallic conductor and the plastic material becomes excessive. It is difficult to control the preheat temperature of a metallic conductor in a manufacturing environment to be within a relatively narrow range in order to meet the above requirements.
Another problem relates to the adhesion of the single layer, irradiation cross-linked insulation to the underlying metallic conductor. It has been found that during the irradiation cross-linking process, the insulation contracts about the metallic conductor. If the outer surface of the metallic conductor does not have a completely regular profile, the adhesion of the insulation is increased and presents problems for craftspersons when they try to strip the insulation from the conductor for field connections.
There is also a problem with respect to twisted pairs of metallic conductors each having an irradiation cross-linked insulation. Normally the cross-linkable polyvinyl chloride composition, prior to irradiation, is softer than a classical thermoplastic polyvinyl chloride composition. If the conductors are twisted together in pairs before the insulation is irradiation cross-linked, the insulation, because it may be highly plasticized, does not exhibit the desired mechanical properties and may be subject to deformation.
Also of concern is the amount of polymeric insulation that is placed over a metallic conductor prior to irradiation cross-linking. Although this insulation is much less costly than a prior style insulation which included a braided covering, it still has a relatively high wall thickness. Therefore in order to irradiation cross-link the polymeric material, a relatively high voltage accelerator is needed. This requires a somewhat high capital investment and the use of concrete vaults in which to perform the cross-linking. It is highly desirable to have a cross-linkable insulation material which covers the conductors and which can be treated by an accelerator on line, and in tandem with the insulation process.
Still another problem arises when using insulation displacement connectors to terminate a conductor which is insulated with a single layer of irradiation cross-linked plastic material. One such connector is a bifurcated beam connector that is shown in U.S. Pat. No. 3,858,158 which issued on May 15, 1974 in the names of R. W. Henn et al. In order to terminate the conventional irradiation cross-linked insulated conductor with a bifurcated beam connector, the insulated conductor must be inserted into a slot of the connector through repetitive steps in order to insure penetration of the insulation by furcations of the split beam portion. Indeed, some commercially available split beam connectors are not approved for making connections with irradiation cross-linked insulated conductors. As a result, these kinds of insulated conductors require more expensive termination arrangements.
One commercially available compositely insulated conductor includes an inner layer of polyolefin insulation which covers a copper conductor and which is surrounded by an irradiated polyvinyl chloride skin. During a stripping operation, it has been found that the removal of the irradiated polyvinyl chloride skin leaves behind the polyolefin inner layer over the copper. This is indicative of a lack of adhesion between the irradiation cross-linked polyvinyl chloride and the polyolefin. The lack of adhesion between the inner and the outer layers creates problems in the field for a craftsperson who, when stripping the insulation, is desirous of removing both the skin and the inner layer. Inasmuch as the inner layer of polyolefin may not be as fire retardant as a polyvinyl chloride composition, the skin must be relatively thick in order to compensate for the lack of fire retardancy of the inner layer. Also, the skin is relatively thick relative to the inner layer in order to obtain required physical properties for the combined layers. This, of course, may require higher voltage accelerators in order to penetrate the relatively thick skin and to obtain a degree of cross-linking necessary to provide the required mechanical properties.
What is desired is a universal type of insulated conductor which can be used in many applications of loose wire such as in central offices, homes, and the like, and which can be wrapped, soldered or used with insulation displacement connectors. What is also required is a method of making such a universal type insulated conductor which has a relatively hard outer surface, which is relatively friction-free to permit pulling of the wire in racks in central offices. The sought after insulated conductor also should be easily manufactured.